| Crystal Springs Dam | |
| Name Official: | Lower Crystal Springs Dam |
| Dam Crosses: | San Mateo Creek |
| Res Name: | Lower Crystal Springs Reservoir |
| Location: | San Mateo County, California |
| Operator: | San Francisco Public Utilities Commission |
| Dam Length: | 600feet |
| Dam Height: | 140feet |
| Dam Width Base: | 40feet |
| Res Capacity Total: | 57910acre.ft |
| Res Catchment: | 29.4sqmi |
| Res Surface: | 1323acres |
| Coordinates: | 37.5285°N -122.3622°W |
Crystal Springs Dam is a gravity dam constructed across the San Mateo Creek in San Mateo County, California. It impounds water to form the Lower Crystal Springs Reservoir which sits atop the San Andreas Fault in a rift valley created by the fault. The dam itself is located about 300 yards (273 meters) east of the fault.[1] [2] It was among the first concrete gravity dams built in the western United States.[3] Skyline Boulevard runs over the dam, which also forms the trailhead of the popular Sawyer Camp Trail.
Spring Valley Water Works, the private utilities corporation that eventually developed the Crystal Springs Dam, was formed in 1858 to provide the rapidly expanding port city of San Francisco with a reliable water source. Under the leadership of George Ensign, a prominent land baron, it quickly established a monopoly on the San Francisco water market.[4] Quickly understanding that any potential water source would have to come outside of the naturally barren city limits, Ensign turned towards adjacent San Mateo County, along with its largely undeveloped mountains and streams. Over the next three decades, Spring Valley Water Works constructed a variety of conduits, dams, and tunnels to connect the San Andreas Valley watershed to San Francisco. Despite this, there was still a desire for a larger reservoir to ensure that San Francisco had a more reliable long-term water source.[5]
Hermann Schussler, a German immigrant who moved to California in 1864 as a prospective civil engineer, quickly rose the ranks to become Chief Engineer of the Spring Valley Water Company. Cognizant of the growing desire for a reservoir, Schussler encouraged the Company to acquire as much watershed property as possible. By the time that Schussler began considering the development of a dam to impound the San Mateo Creek, the Company had already acquired over 100000acres of prime property across the San Francisco Peninsula.
The location of the proposed dam site had its benefits and downfalls: located in a 700feet valley formed by San Mateo Creek, sandstone cliff walls rose over 200feet from the creekbed. The dam encloses a larger valley that spreads outwards both east and west of the dam itself. Schussler's initial design created a lake approximately 9miles long. Geological testing was conducted using boring techniques, which disclosed that the valley was favorable towards the dam's construction: the foundation was hard blue sandstone that contained no cracks nor fissures.
It quickly became clear that the scale of the hypothesized dam would be large, as Schussler's designs called for a projected height of 140feet; at the time there were only 17 constructed dams in the world taller than 95feet, and all 17 of them were located outside of the United States. Furthermore, a majority of these dams were either rock-filled or earth-filled dams. This approach, however, could not work for Schussler's design, as there were no suitable quarries to procure materials from. However, during the search, a large outcrop of indurated sandstone was located, especially conductive for concrete. As such, Schussler switched to a design using mass concrete blocks.[6]
At the time, the United States had no large-scale manufacturer of cement, leading to the Portland cement being transported via ship from Swanscombe,England, which at the time was the biggest Portland cement manufacturing plant in the United Kingdom.[7] The dam was constructed by separately pouring large blocks of the structure in place, and allowing them to set before pouring the adjoining blocks. This enabled the bricks to set and harden individually, while also ensuring that the concrete would not crack during the curing process. An important design feature is that neither the horizontal nor the vertical joints line up. This helps the structure act as a single monolithic construction despite being made of thousands of individual blocks, while also enhancing its structural stability.
Schussler ensured that each block of the dam was poured with concrete made to the exact proportions of his mix specifications.
The structure was completed in 1888 at a height of 120feet. However, a wet winter in 1889-1890 led to the dam overflowing, leading to an addition to bring the total height of the dam to 145feet, making it the tallest dam in the United States at the time.[8] Furthermore, at the time of its completion, with 4239000cuft of concrete, it was the largest concrete structure in the world.[9]
In 1976, the dam was designated as a California Historic Civil Engineering Landmark by the American Society of Civil Engineers (ASCE).[10] [11] The dam was later designated as a National Historic Civil Engineering Landmark by ASCE in 2023.[12]
The dam has survived both the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake – despite its location about east of the San Andreas Fault. It was subject to severe shaking in both earthquakes.[3]
The San Francisco Public Utilities Commission began the process of renovating the dam in 2003. A major step in the renovation process, doubling the width of the main spillway and raising the dam to increase the water storage capacity, was completed in 2012. The renovations, which are intended to improve the reliability of the system in the event of an earthquake, were completed in 2016. Skyline Boulevard, the road on top of the dam, was closed for the construction of a new replacement bridge in October 2010. Construction completed in late 2018 and the roadway was reopened in January 2019.[13] [14]
The bridge closed again to vehicular traffic on February 10, 2020 for minor repairs to the parapet wall, reopening on March 16, 2020.[15]